Well-defined poly(L-lactide)-b-poly(ethylene oxide) (PLLA-b-PEO) copolymers with different branch arms were synthesized via the controlled ring-opening polymerization Of L-lactide followed by a coupling reaction with carboxyl-terminated poly (ethylene oxide) (PEO); these copolymers included both star-shaped copolymers having four arms (4sPLLA-b-PEO) and six arms (6sPLLA-b-PEO) and linear analogues having one arm (LPLLA-b-PEO) and two arms (2LPLLA-b-PEO). The maximal melting point, cold-crystallization temperature, and degree of crystallinity (X,) of the poly (L-lactide) (PLLA) block within PLLA-b-PEO decreased as the branch arm number increased, whereas X, of the PEO block within the copolymers inversely increased. This was mainly attributed to the relatively decreasing arm length ratio of PLLA to PEO, which resulted in various PLLA crystallization effects restricting the PEO block. These results indicated that both the PLLA and PEO blocks within the block copolymers mutually influenced each other, and the crystallization of both the PLLA and PEO blocks within the PLLA-b-PEO copolymers could be adjusted through both the branch arm number and the arm length of each block. Moreover, the spherulitic growth rate (G) decreased as the branch arm number increased: G(6SPLLA-b-PEO) < G(4sPLLA-b-PEO) < G(2LPLLA-b-PEO) < G(LPLLA-b-PEO). (c) 2006 Wiley Periodicals, Inc.